OpenPLL

OpenPLL is an integer-N bang-bang digital PLL implementation targeting Sky130. The current high-frequency source/integration path is IntegerPLL_HardMacroTop_EINVP, which combines the digital core RTL, filled BBPD macro, physical IntegerPLL_DCO_EINVP_COARSE oscillator macro source, and hard-top hardening flow. The current release promotes the all-HD coarse-DCO source and 25 MHz configured RTL/behavioral path, with regenerated hard-top and configured-wrapper post-layout artifacts passing the release gate. The DCO is one macro and one oscillator loop: HD NAND/NAND2B turn/pass mirror delay provides coarse band selection, while 255 HD NAND2 load cells provide local fine tuning. It does not use parallel DCO macros, a mux-selected feedback tap, or a NOT-chain ring.

The DCO is treated as a PLL supply island. Its macro has a local met4/met5 PDN/ring, the hard top stitches both VPWR and VGND into that island, and the parent hard-top route blocks met3/met4/met5 signal routing over the DCO macro body.

The ring-facing output buffer intentionally stays sky130_fd_sc_hd__buf_1 to limit oscillator-node loading. The fine-load digital enable is NAND2 pin B, the pin whose series NMOS is directly connected to VGND in the Sky130 HD nand2_1 SPICE view. If more oscillator speed margin is needed, adjust the ring/mirror gate drive, effective loop length, or fine-load topology; do not upsize the ring-facing output buffer as a speed fix because that directly increases oscillator load.

Older IntegerPLL_DCO_EINVP and IntegerPLL_DCO_EINVP_SPARSE72 artifacts are retained as low-frequency and 200 MHz diagnostic history, not as the current 100/250/300/400/500 MHz hardtop target history.

What to Read

• PLL_ARCHITECTURE.md describes the PLL architecture and block-level control conventions.
• SKY130_IMPLEMENTATION.md summarizes the Sky130 implementation, physical status, signoff evidence, and local build commands.
• SPICE_VALIDATION.md records the transistor-level validation boundary, promoted evidence, and non-promoted diagnostic history.
• REPRODUCIBILITY.md describes the Apptainer/SingularityCE flow for reproducing the latest release gate.

Current Status

The current fast coarse-DCO source/behavioral gates are:

make -C OpenPLL check-sky130-macros
make -C OpenPLL check-pll-25mhz-divider-config
make -C OpenPLL check-pll-25mhz-divider-controller
make -C OpenPLL check-pll-25mhz-configured-wrapper
make -C OpenPLL check-pll-25mhz-configured-behavioral

The latest-release result can also be audited or rebuilt inside Apptainer:

make -C OpenPLL apptainer-build
make -C OpenPLL apptainer-audit-release
make -C OpenPLL apptainer-rebuild-release

rtl/IntegerPLL_25MHzModeConfig.v is the reusable preset table for a 25 MHz reference. It exposes the restored /4, /10, /12, /16, and /20 settings:

Feedback divider	Target	Coarse	Fine seed	KI	KP
4	100 MHz	C20	93	16	4
10	250 MHz	C06	234	16	4
12	300 MHz	C04	90	16	4
16	400 MHz	C02	76	16	4
20	500 MHz	C01	121	16	4

The table also emits a 10-bit DLF_Ext_Data seed equal to target_code << 2 and CONFIG_VALID. Unsupported divider values are passed to the lower divider bus for observability but hold CONFIG_VALID=0 and do not enter tracking.

rtl/IntegerPLL_25MHzModeController.v and rtl/IntegerPLL_HardMacroTop_EINVP_25MHzConfigured.v are the intended configured RTL entry point for the 25 MHz reference path. The external control is the feedback-loop divider value, FEEDBACK_DIVIDER[4:0], plus PLL_ENABLE; the controller latches the divider, applies the matching characterized coarse/fine/gain seed when CONFIG_VALID=1, asserts DLF_Clear for the preset seed, and then enables closed-loop tracking. The shipped hard-macro path uses the divider-clocked DLF macro; the generic digital core and IntegerPLL_Top still support DLF_UPDATE_ON_PLLOUT=1 as a diagnostic build option if the digital-core macro is rebuilt for that variant. make -C OpenPLL check-pll-25mhz-configured-wrapper runs the wrapper-level divider sequencing test and checks that those preset controls reach the hard-macro instance. The configured wrapper also has a hardening flow as IntegerPLL_HardMacroTop_EINVP_25MHzConfigured. The regenerated wrapper artifact passes Magic/KLayout DRC, LVS, setup/hold, max slew/cap, and the configured hard-top release checker. The lower-level IntegerPLL_HardMacroTop_EINVP pins remain available for characterization and custom bring-up.

Next-version BBPLL work should focus on the broader control extension needed for robust acquisition from arbitrary phase and code, rather than on packaging the configured interface.

make -C OpenPLL check-pll-25mhz-configured-behavioral runs reset-to-tracking behavioral PLL regressions for all restored 25 MHz divider values. It uses the real controller, digital core, divider, and BBPD with a behavioral DCO table fitted to the all-HD 255-load coarse-DCO measurements, and checks measured output frequency plus non-rail DCO control after the preset load.

The current configured-mode 25 MHz-reference PLL target gate is:

make -C OpenPLL xyce-pll-mixed-signal-25mhz-targets

The current all-HD DCO target table is:

Target	Coarse	Seed	Target-code evidence
100 MHz	C20	93	Interpolated code estimate 93.429
250 MHz	C06	234	Measured target code
300 MHz	C04	90	Interpolated code estimate 89.631
400 MHz	C02	76	Interpolated code estimate 76.037
500 MHz	C01	121	Interpolated code estimate 120.677

Fast deterministic BBPLL jitter sweeps are under build/pll_jitter_25mhz_allhd255/ with matched v6 rows under build/jitter_compare_25mhz_v6_matched/. They use an ideal BBPD/control model and exclude device noise, supply noise, and extracted interconnect noise. The all-HD DCO improves the worst-case peak-to-peak period jitter screen at all five configured targets versus v6. Fitted TIE RMS is mixed, improving at 100, 300, and 500 MHz while regressing at 250 and 400 MHz, so this remains a deterministic model screen rather than final jitter signoff.

The current fast source/behavioral check for that 25 MHz coarse-DCO path is:

make -C OpenPLL check-pll-25mhz-divider-config check-pll-25mhz-divider-controller check-pll-25mhz-configured-wrapper check-pll-25mhz-configured-behavioral check-sky130-macros

It checks the coarse-DCO RTL shape, including the buf_1 output-buffer constraint, the restored 25 MHz divider presets, the divider controller/wrapper, and the configured behavioral PLL reset-to-tracking regression.

The heavier make -C OpenPLL check-sky130-pll-25mhz-release target checks the current post-layout hard-macro artifacts, configured-wrapper artifacts, target-code rows, and direct-RCX integration summaries.

An optional slow direct-RCX integration smoke for the hardest divider target is:

make -C OpenPLL xyce-pll-postlayout-dco-mixed-25mhz-500m-hold-smoke

It runs the extracted coarse DCO and extracted BBPD in Xyce at C01/code121 with the digital divider/filter in the mixed-signal driver. Its short ADC-sampled frequency estimate is intentionally loose; regenerated standalone RCX DCO rows must remain the precise frequency evidence.

An even slower all-mode direct-RCX near-seed code-update diagnostic is:

make -C OpenPLL xyce-pll-postlayout-dco-mixed-25mhz-nearseed-smokes

The current all-mode direct-RCX run checks +/-4 fine-code starts for all five divider targets with KI=16, KP=4, and FRAC=2. That evidence remains a near-seed direct-RCX integration diagnostic; standalone target-code rows remain the precise frequency evidence.

The legacy low-frequency v1 artifact audit remains available:

make -C OpenPLL validate-sky130-pll-artifacts

It checks the older Sky130 signoff, SPICE, RTL, mixed-signal, and extracted-loop artifacts and writes consolidated summaries under OpenPLL/build/sky130_pll_validation/. The heavier make -C OpenPLL validate-sky130-pll target regenerates those legacy promoted artifacts before running the same audit.

An optional pre-layout mirror-delay diagnostic is:

make -C OpenPLL check-dco-einvp-coarse-mirror-targets

It uses the same small output-buffer-drives 1 assumption. The legacy target is still useful for topology experiments, but the current release DCO source uses an all-HD ring and all 255 thermometer load controls.

Environment

The Makefile first looks for the newer Ciel/Volare Sky130 tree at $HOME/.volare/ciel/sky130, using the newest versions/* entry when there is no direct sky130A symlink. It falls back to $HOME/.volare. Override these variables for another local setup:

export CIEL_SKY130_ROOT=$HOME/.volare/ciel/sky130
export PDK_ROOT=$HOME/.volare/ciel/sky130
export PDK=sky130A
export STD_CELL_LIBRARY=sky130_fd_sc_hd
export LIBRELANE_ROOT=$HOME/src/librelane
export XYCE_MPI_ROOT=$HOME/.local/xyce-mpi

Use make -C OpenPLL check-pdk-stdcell to print and validate the selected PDK. If the shell exports either the legacy default PDK_ROOT=$HOME/.volare or the Ciel registry root PDK_ROOT=$HOME/.volare/ciel/sky130, the Makefile and direct script defaults resolve it to the usable Ciel PDK root; pass PDK_ROOT=... on the make command line or export a non-default root to select something else. The local Ciel install used for this tree includes sky130_fd_sc_hd and sky130_fd_sc_hs reference views. The current coarse high-frequency DCO macro explicitly uses HD cells.

LIBRELANE_ROOT is auto-detected from nearby checkout locations when possible. XYCE defaults to the first Xyce on PATH; MPI/KLU diagnostics use $XYCE_MPI_ROOT/bin/Xyce when that binary exists.

Validation Boundary

The strongest current high-frequency evidence is the post-layout coarse-DCO target map plus clean physical hardtop/SPICE interface checks. The 25 MHz-reference mixed-signal flow is configured-mode polarity and gain evidence around measured target settings; it is not final post-layout PLL lock signoff.

For the 25 MHz reference / 200 MHz fast path, run xyce-pll-postlayout-calibrated-dco-mixed-fast200-sparse72-lock after the sparse72 DCO signoff/RCX/probe targets. That mixed target uses the filled BBPD RCX in Xyce and a DCO phase model calibrated from the sparse72 post-layout RCX frequency points; the current bounded run converges from code 0 to code 192 at 202.314 MHz and from code 255 to code 191 at 196.767 MHz, both within the configured 4 MHz tolerance around the 200 MHz target. The full extracted-DCO C-interface path is retained as a slow diagnostic. The bounded direct-RCX companion is xyce-pll-postlayout-dco-mixed-fast200-sparse72-near-lock-motion: it keeps both the filled BBPD and sparse72 DCO RCX decks in Xyce, uses Xyce's mixed-step API for post-decision DCO-code updates, and drives REF as a 25 MHz pulse source for robust BBPD edge capture. The current sparse72 diagnostic run measures fixed code 196 at 199.734 MHz, pulls a low-side start from code 184 to 197 with measured PLLOUT at 200.000 MHz, and pulls a high-side start from code 220 to 194 with measured PLLOUT at 200.000 MHz. This is direct-RCX near-lock evidence, not a full rail-to-rail extracted-DCO regression. Running the current C-interface executables under mpirun is not a practical speedup path: the probed mpirun -np 2 smoke reached Xyce completion but left both driver ranks stuck instead of exiting cleanly.

For the 100 MHz-order fast path, xyce-pll-mixed-signal-fast100-coarse4-smoke is a bounded C-interface check with the filled BBPD RCX in Xyce and a behavioral fast DCO table plus independent coarse offset in the compiled driver. It is useful local polarity/range evidence, not physical fast-DCO signoff. xyce-pll-analog-dco-mixed-fast100-coarse4-acq is the stricter mixed-signal fast-path check: Xyce owns the filled BBPD RCX, behavioral analog DCO phase, reference, and divider, while the compiled driver owns only the DLF code update. The current tuned bounded run reaches fine code 34 from 0 and code 30 from 64 around the 126.88745 MHz target, with measured PLLOUT frequencies of 127.389 MHz and 126.316 MHz respectively.

Diagnostic targets that are slow, host-sensitive, or known not to pass from both rails are deliberately excluded from the promoted artifact gate and are documented as diagnostic history in SPICE_VALIDATION.md.